Endpoint detection by chemical reaction and photoionization

ABSTRACT

Detection of the endpoint for removal of a target film overlying a stopping film by removing the target film with a process that selectively generates a chemical reaction product (for example ammonia when polishing a wafer with a nitride film in a slurry containing KOH) with either the target or stopping film, and monitoring the level of chemical reaction product by threshold photoionization mass spectroscopy as the target film is removed.

RELATED APPLICATIONS

This invention is related to the following copending U.S. patentapplications:

Ser. No. 09/073,602,entitled “Endpoint Detection by Chemical Reaction”;

Ser. No. 09/073,601, entitled “Endpoint Detection by Chemical Reactionand Light Scattering”;

Ser. No. 09/073,605, entitled “Indirect Endpoint Detection by ChemicalReaction”; now U.S. Pat. No. 6,066,504

Ser. No. 09/073,604, entitled “Indirect Endpoint Detection by ChemicalReaction and Chemiluminescence”; now U.S. Pat. No. 6,126,848

Ser. No. 09/073,607, entitled “Endpoint Detection by Chemical Reactionand Reagent”; and

Ser. No. 09/073,603, entitled “Reduction of a Gaseous Product inSolution”;

all filed on the same day, all assigned to the present assignee, and allincorporated by reference in their entireties.

FIELD OF THE INVENTION

This invention is directed to semiconductor processing and moreparticularly to the detection of the endpoint for removal of one filmoverlying another film.

BACKGROUND OF THE INVENTION

In the semiconductor industry, critical steps in the production ofintegrated circuits are the selective formation and removal of films onan underlying substrate. The films are made from a variety ofsubstances, and can be conductive (for example metal or a magneticferrous conductive material) or non-conductive (for example aninsulator). Conductive films are typically used for wiring or wiringconnections. Nonconductive or dielectric films are used in severalareas, for example as interlevel dielectrics between layers ofmetallization, or as isolations between adjacent circuit elements.

Typical processing steps involve: (1) depositing a film, (2) patterningareas of the film using lithography and etching, (3) depositing a filmwhich fills the etched areas, and (4) planarizing the structure byetching or chemical-mechanical polishing (CMP). Films are formed on asubstrate by a variety of well-known methods, for example physical vapordeposition (PVD) by sputtering or evaporation, chemical vapor deposition(CVD), and plasma enhanced chemical vapor deposition (PECVD). Films areremoved by any of several wellknown methods, for example CMP, dryetching such as reactive ion etching (RIE), wet etching, electrochemicaletching, vapor etching, and spray etching.

It is extremely important with removal of films to stop the process whenthe correct thickness has been removed (the endpoint has been reached).With CMP, a film is selectively removed from a semiconductor wafer byrotating the wafer against a polishing pad (or rotating the pad againstthe wafer, or both) with a controlled amount of pressure in the presenceof a slurry. Overpolishing (removing too much) of a film results inyield loss, and underpolishing (removing too little) requires costlyrework (redoing the CMP process). Various methods have been employed todetect when the desired endpoint for removal has been reached, and thepolishing should be stopped.

The prior art methods for CMP endpoint detection suitable for all typesof films involve the following types of measurement: (1) simple timing,(2) friction by motor current, (3) capacitive, (4) optical, (5)acoustical, and (6) conductive.

An exception to the above is U.S. Pat. No. 5,399,234 to Yu et al, inwhich a chemical reaction is described between potassium hydroxide inthe polishing slurry and the layer being polished. The endpoint forpolishing is monitored by sending acoustic waves through the slurry anddetecting changes in the acoustic velocity as the concentration ofreaction product (thought to be silanol in the case of polishing silicondioxide) from the layer being polished decreases upon reaching anunderlying polish stop layer.

These prior art methods each have inherent disadvantages such asinability for real-time monitoring, the need to remove the wafer fromthe process apparatus for examining completion of polishing (notin-situ), or a lack of sensitivity.

These disadvantages have been overcome with an in-situ endpointdetection scheme for conductive films as described in U.S. Pat. No.5,559,428 to Li et al titled “In-Situ Monitoring of the Change inThickness of Films,” however a suitable endpoint detection fornon-conductive films has yet to be described.

Thus, there remains a need for an in-situ, real-time endpoint detectionscheme suitable for use with all types of films. Such a scheme shouldhave high detection sensitivity and extremely fast response time,preferably less than 1 or 2 seconds.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand apparatus for detecting the endpoint for removal of any type of filmoverlying another film.

Another object of the present invention is to provide for in-situendpoint detection as the film is being removed (i.e. real-time).

Yet another object is to provide endpoint detection with high detectionsensitivity and extremely fast response time.

In accordance with the above listed and other objects, a method fordetecting the endpoint for removal of a target film overlying a stoppingfilm, by removing the target film with a process that selectivelyproduces a gaseous chemical reaction product with one of the stoppingfilm and the target film; and monitoring by threshold photoionizationmass spectrometry the level of chemical reaction product as the targetfilm is removed, is described.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages will be more readilyapparent and better understood from the following detailed descriptionof the invention, in which:

FIG. 1 shows a cross-section of a target film to bechemically-mechanically polished;

FIG. 2 shows a cross-section of an ammonia scrubber for reducingpre-polish ammonia concentration in the slurry;

FIG. 3 shows a cross-section of an extraction unit for extractingammonia gas from the slurry;

FIG. 4 shows the setup for chemical detection using fourier transformmicrowave spectroscopy; and

FIG. 5 shows the setup for chemical detection using thresholdphotoionization mass spectroscopy; all in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described herein in the context ofchemical-mechanical polishing merely as a specific example, and is notmeant to limit applicability of the invention to semiconductortechnology. Those skilled in the art will understand that the inventionis broadly applicable to any process in which it is desirable to detectthe endpoint for removal of a target film overlying a stopping film, byremoving the target film with a process that selectively produces agaseous chemical reaction product with one of the stopping film and thetarget film; and monitoring by threshold photoionization massspectroscopy the level of chemical reaction product as the target filmis removed.

As shown in FIG. 1, we have discovered that when chemically-mechanicallypolishing a substrate 100 with a target film of oxide (SiO₂) 104 over astopping film of nitride (Si₃N₄) 102 with a slurry containing potassiumhydroxide (KOH), a chemical reaction occurs when the interface 106 isreached, resulting in the production of ammonia (NH₃). Morespecifically, the slurry used is a mixture of fumed silica, water, andKOH, with a pH of about 10.5. When p olishing oxide, the followingreaction occurs:

SiO₂+2KOH+H₂O→K₂SiO₃+2H₂O

When polishing nitride, the following reaction occurs:

Si₃N₄+6KOH+3H₂O→3K₂SiO₃+4NH₃

The ammonia produced is dissolved in the slurry, and because of therelatively high pH it exists primarily in the form of NH₃ rather thanNH₄ ⁺. Thus the change of ammonia level in the slurry indicates that theunderlying nitride film has been reached, and the endpoint for removalof the oxide film can be determined by monitoring the level of ammoniain the slurry. Once the endpoint is reached, the polishing can bestopped.

More generally, the endpoint for removal of any non-nitride-containingfilm overlying a nitride-containing film can be detected by monitoringthe level of ammonia in the slurry. Conversely, the endpoint for removalof a nitride-containing film overlying a non-nitride-containing film canalso be detected in a similar manner, with ammonia being presentinitially and a marked decrease in the amount of ammonia indicating theendpoint.

Even more generally, the endpoint for removal of any film overlyinganother film can be detected by monitoring the level of a chemicalreaction product in the slurry as a component of the slurry reactsselectively with one of the films (either the overlying or underlyingfilm).

The reaction described above producing ammonia will be discussed asfollows but is not intended to limit the scope of the invention to thatparticular embodiment.

In order to implement the above discovery concerning the production ofammonia in an environment suitable for manufacturing, in-situ (i.e.while the wafer is being polished) slurry collection and sampling isrequired. Preferably, the collection and sampling provide a rapidresponse with high sensitivity (to ammonia) and minimizes the effect ofinterference from other substances in the slurry and in the surroundingair.

Unfortunately, the slurry described above already contains ammonia priorto being used for polishing. The ammonia concentration varies from aslow as 5.0×10⁻⁶ Molar (M) to as high as 5.0×10⁻⁵ M. The ammonia producedin the slurry when polishing a blanket (i.e. uniform) layer of nitrideis approximately 1.0×10⁻⁴ M at room temperature; for a typical lowpattern factor production wafer having a nitride layer which covers 15%of the wafer area (the rest being oxide), polishing at the interface ofthe oxide/(oxide+nitride) produces 1.5×10⁻⁵ M. In this case, the desiredconcentration change will not be able to be distinguished from thefluctuation of the pre-polish ammonia concentration. Therefore theammonia concentration prior to polishing this type of wafer must bereduced in order to achieve the desired sensitivity.

The reduction in pre-polish ammonia concentration can be achieved ifnecessary by using an ammonia scrubber. In the above case, the scrubberreduced the concentration to approximately 2.5×10⁻⁶ M. The maincomponent of the scrubber is a Liqui-Cel Membrane Contactor 200 (modelExtra-Flow 4×28 made by Hoechst Celanese) shown in FIG. 2. The Contactorcontains Celgard (TM) microporous polypropylene fibers 202 which arehydrophobic and do not allow water-based solutions to penetrate throughthe fiber membranes, but do allow gas exchange. Slurry from a reservoirenters contactor 200 at 204 and flows through contactor 200 on theoutside of the fibers (also called shellside) allowing ammonia topermeate to the inside of the fibers (also called lumenside) beforeexiting at 206 and recirculating back to 204. To facilitate removal ofthe ammonia in the slurry, an aqueous HCl solution from anotherreservoir with a pH of approximately 3 is circulated in the lumenside,entering at 208 and exiting at 210 before recirculating back to 208.Ammonia gas from the slurry crossing into this HCl stream is immediatelyconverted to NH₄ ⁺ by a moderately high concentration of protons,therefore effectively preventing a possible buildup of any appreciableamount of NH₃ in the lumenside of the scrubber. The recirculatingaqueous HCl stream and reservoir can then be the sink for a large amountof ammonia from the slurry.

With an aqueous HCl reservoir of approximately 10 liters of wateradjusted to a pH of 3.5 using 0.1 M HCl solution, and 10 liters of1.0×10⁻⁴ M ammonia solution at a pH of 10.7 passing through theabove-described contactor, the ammonia level was reduced to the desired2.5×10⁻⁶ M in 30 minutes. The time can be reduced by increasing the sizeof the contactor, using several contactors in series, or gently heatingthe slurry to increase the volatility of the ammonia, or any combinationof the three. The desired target ammonia level in the slurry can bemeasured by a commercially available detector such as an ammoniaspecific ion selective electrode (ISE).

Once the slurry has reached the desired target ammonia level, it is usedto polish a wafer. The slurry is collected from the polishing pad forsampling during the polishing process.

Sampling Methods for Ammonia Dissolved in Slurry

Once the slurry is collected from the polishing pad, various samplingmethods can be used. Representative methods are described as follows.

Ion Selective Electrode

One way of detecting liquid ammonia is by using a commercially availableion selective electrode, in which an internal electrode solution with agiven pH is in contact with the solution of interest through asemi-permeable hydrophobic membrane. The membrane allows ammonia gasfrom the slurry to pass through, changing the pH of the internalelectrode solution. The pH is monitored electrochemically. This methodcan detect ammonia down to 5×10⁻⁷ Molar, and in less than 30 seconds athigh concentrations, but at low concentrations can take as long as 1-3minutes, which is suitable for many applications but not for thisparticular CMP endpoint detection as desired.

Fluorometric Measurement

Another way of detecting ammonia in a liquid phase is by using a complexreagent which renders a fluorescent species upon contact with theammonia. The reagent used contains O-phthaldialdehyde (OPA) and sulfiteacting as a reducing agent. (See Z. Genfa et al, “FluorometricMeasurement of Aqueous Ammonium Ion in a Flow Injection System” Anal.Chem., Vol 61, page 408, 1989) The reaction product is highlyfluorescent and can be detected using emission spectroscopy at aconcentration down to 2×10⁻⁸ Molar. However, this method can take up to10 minutes to reach completion, which is again not suitable for this CMPapplication.

Chemiluminescence

Measurement of chemiluminescence due to the reaction between ammonia andhypobromite in an alkaline solution using a photomultiplier with anamplifier has also been performed (See X. Hu et al, “Determination ofAmmonium Ion in Rainwater and Fogwater by Flow Injection Analysis withChemiluminescence Detection: Anal. Chem., Vol. 65, page 3489, 1993).This method, however, does not have the required sensitivity formonitoring ammonia generated in this application while polishing nitridewith CMP. The response time of this method is too slow for true in-situreal time process control.

Extraction of Ammonia Gas from the Slurry

In order to detect and monitor ammonia in a gaseous form, thus enablingmethods such as mass spectroscopy, slurry from the polishing apparatus(not shown) is pumped through an ammonia extraction unit 300 shown inFIG. 3. Extraction unit 300 is constructed from polypropylenemicroporous hollow fibers 302 obtained from a dismantled Liqui-CelContactor (model 2.5×8 made by Hoechst Celanese). Fibers 302 allow gasbut not liquid to pass from the outside to the inside of the fibers.

Slurry is pumped in at 304 through extraction unit 300 on the outside offibers 302 and exits at 306. Dry air (from a drier containing an ammoniafilter) is pumped in at 308 through the inside of the fibers and exitsas stream 312 containing ammonia gas at 310. The dry air is pumped at areduced pressure of approximately 30 Torr to facilitate ammoniatransport from the slurry through the fibers and into the air stream.The reduced pressure also increases the overall flow velocity, whichhelps to reduce the response time for measuring the change in ammoniaconcentration.

Analysis Methods for Ammonia Gas

The ammonia-containing gas stream 312 from extractor 300 is thenanalyzed and monitored for endpoint detection for removal of the targetfilm. Gas phase chemical analysis, such as standard mass spectroscopycan be highly sensitive and have a fast response time, which would bedesirable for endpoint detection. However, with slurry sampling, thereis substantial interference from water vapor which is only 1 atomic massunit (AMU) higher and present in abundance. During the electron impactionization, water of mass 18 can lose a hydrogen resulting in a OH ionwith mass 17, which has a mass identical to NH3⁺. Thus the ammoniasignal from the slurry can be very effectively masked, and endpointdetection becomes impossible. The following techniques solve the maskingproblem.

Fourier Transform Microwave Spectroscopy (FTMS)

Fourier transform microwave spectroscopy using pulsed gas phase sampleinjection has extremely high detection sensitivity and chemicalselectivity, and can accurately measure contaminants at a sub ppb level,and overcomes the masking problem associated with mass spectrometry (SeeHarmony et al, “A Compact Hot-Nozzle Fourier Transform MicrowaveSpectrometer” Rev. Sci. Instrum., Vol. 66, page 5196 (1995), alsoSuenram et al, “Effect of Tunneling Motions on the Quadrupole HyperfineStructure of Hydrazine” J. Mol. Spectrosc., Vol. 137, page 127 (1989)).

The ammonia-containing gas stream 312 from extractor 300 (shown in FIG.3) is directed to a Fourier transform microwave spectrometer 400 shownin FIG. 4. A bypass valve 402 is used to regulate the flow of the streaminto a pulsed valve 404. Bypass valve 402 allows a substantial overallflow through extraction unit 300 resulting in a rapid measurementresponse time without overloading spectrometer 400 with sampling gasfrom the incoming ammonia-containing air stream.

Pulsed valve 404 injects sample gas from stream 312 into thespectrometer vacuum chamber with a repetition rate of approximately 20Hz and a valve open time of 1 ms. The pulsed gas enters the chamber andexpands supersonically, thereby cooling to about 10 degrees K. Highintensity microwave radiation (23.786 GHz for ammonia) is pulsed also at20 Hz (timed to coincide with the pulsed gas) into the chamber frommicrowave source 406 to a transmitter/receiver 408. The ammoniamolecules absorb the radiation and re-radiate in a free induction decayprocess. The decay is then digitized and transformed by detector 410 toobtain a frequency domain signal. The intensity of the signal ismonitored to determine the amount of ammonia present in the slurry.

Threshold Photoionization Mass Spectroscopy

In this method, threshold photoionization of the ammonia with subsequentdetection by a quadropole mass analyzer is used as shown in FIG. 5. Theammonia-containing gas stream 312 exiting from extractor 300 shown inFIG. 3 is directed through a bypass valve 502 to a photoionization massspectrometer 500. A sample region 504 at a pressure of near 1 Torr isirradiated with a krypton resonance lamp 506 with two primary vacuumultraviolet lines at 10.0 eV and 10.6 eV. The line at 10.6 eV isslightly above the ammonia ionization potential (IP) of 10.18 eV andnicely below the IP of water at 12.6 eV. The ions resulting from theirradiation are directed through a small orifice into a high vacuumchamber 508 and detected with a modified quadrupole mass spectrometer510. The mass analysis is used to discriminate against interference fromhydrocarbon species with low ionization potentials which can be readilyionized by the krypton lamp. A detection sensitivity of better than 10ppb is expected.

An added advantage of using threshold photoionization mass spectroscopyinstead of more conventional electron impact ionization is theprevention of water fragmentation which gives rise to fragment ions (OH)at mass 17, which is the same mass as ammonia. Additionally, the photonenergy is below the ionization potentials of other possible gas phasemolecules which might be present including, N₂ O₂ CO₂, CO, and, mostimportantly, H₂O.

Note that the threshold photoionization mass spectroscopy and Fouriertransform microwave spectroscopy methods and apparatus described are notrestricted to use with monitoring the endpoint for CMP. If an overlyingfilm is being removed from an underlying film by etching, for exampledry etching (e.g. reactive ion etching), an underlying film (i.e. etchstop) may be selected which generates a marker chemical reaction productupon contact with the etchants. The reaction products of the etchingprocess can be sampled by any of these methods in order to monitor thelevel of the marker chemical reaction product.

The photoionization and Fourier transform microwave spectroscopy methodsmay also be used to detect a substance such as ammonia at very low (e.g.1.0×10⁻⁵ M or lower) concentrations in a liquid, by extracting thechemical present in a gaseous form from the liquid, and employing thesteps described above to detect the ammonia in gaseous form.

In summary, methods and associated apparatus have been described whichare capable of detecting the endpoint for removal of any type of filmoverlying another film. The present invention provides for in-situendpoint detection as the film is being removed, and with high detectionsensitivity and extremely fast response time.

While the invention has been described in terms of specific embodiments,it is evident in view of the foregoing description that numerousalternatives, modifications and variations will be apparent to thoseskilled in the art. Thus, the invention is intended to encompass allsuch alternatives, modifications and variations which fall within thescope and spirit of the invention and the appended claims.

What is claimed is:
 1. A method for detecting the endpoint for removalof a target film overlying a stopping film in a chemical-mechanicalpolishing process using a slurry, the method comprising the steps of:reducing a concentration of a gaseous component in the slurry; aftersaid reducing step, removing the target film with thechemical-mechanical polishing process using the slurry, where theprocess selectively produces said gaseous component in the slurry as achemical reaction product with one of the stopping film and the targetfilm; extracting the chemical reaction product as a gas from the slurry;and after said extracting step, monitoring by threshold photoionizationmass spectroscopy the level of chemical reaction product as the targetfilm is removed.
 2. The method of claim 1 wherein the extraction stepcomprises: contacting a first side of a hydrophobic membrane with theslurry; contacting a second side of the hydrophobic membrane with a gasstream; and allowing the chemical reaction product to pass through themembrane as a gas and become entrained in the gas stream.
 3. The methodof claim 1 wherein the gaseous chemical reaction product is ammonia. 4.The method of claim 1 further comprising the step of stopping theremoval process when the endpoint has been reached.
 5. The method ofclaim 1 wherein the process selectively produces a gaseous chemicalreaction product with the stopping film.
 6. The method of claim 1wherein the amount of gaseous chemical reaction product increases as thetarget film is removed.
 7. The method of claim 1 wherein processselectively produces a gaseous chemical reaction product with the targetfilm.
 8. The method of claim 1 wherein the amount of gaseous chemicalreaction product decreases as the target film is removed.